1. Field of the Invention
The present invention relates to an alignment apparatus having sensors capable of detecting the positions of a substrate with a resolution of nano meter (nm) order. More particularly, the invention relates to an apparatus for aligning a mask or reticle and a photosensitive substrate, which is suited for projection type aligners (stepper-aligner), a proximity type aligner, and others used for fabricating semiconductor devices or liquid crystal display elements.
2. Related Background Art
In recent years, in the lithography processes of semiconductor element fabrication, the step-and-repeat type reducing projection aligner (stepper) has been widely used as an apparatus for transferring the reticle pattern to a wafer with a high resolution. This has been accompanied by the evolution of higher integration of semiconductor devices, the development of shorter light wavelengths for exposure light and projection lenses having higher aperture number (N.A.) for a stepper of the kind. Recently, the resolution line width on the wafer has reached a sub-micron order (approximately 0.5 to 0.6 .mu.m). In order to transfer such a high resolution image pattern as this, the precision of alignment (superposition) which corresponds to such resolution power is required. Therefore, it is conceivable that the alignment precision can be enhanced by increasing the detection resolution of an alignment sensor, for example.
A high resolution alignment sensor of the kind is disclosed in U.S. Pat. No. 4,710,026, for example. In this alignment sensor, one-dimensional interference fringes are produced on a diffraction grating mark by irradiating coherent parallel beams in two directions which are different from each other to the one-dimensional diffraction grating mark formed on the wafer. Further, there is proposed a system in which the intensity of the diffraction light (interference light) from the diffraction grating mark by the irradiation of the interference fringes is detected photoelectrically.
For this disclosed system, there are the heterodyne method providing a given frequency difference for the parallel beams in two directions and the homodyne method which does not provide any frequency difference. In the homodyne method, static interference fringes are produced in parallel to the diffraction grating mark, and when a position is detected, it is necessary to move the diffraction grating mark (object) finely in the direction of its pitches. The mark position is obtained with the interference fringes as reference. On the contrary, the interference fringes run at a high speed in the direction of its fringes (pitch direction) at the beat frequency in the heterodyne method because of the laser beam frequency difference (beat frequency) therein, and the mark position is not obtained with the interference fringes as reference, but it is obtained with the time element (phase difference) as reference, which mainly follows the high-speed shifting of the interference fringes.
For example, in the heterodyne method, by way of giving the frequency difference, a phase difference (within .+-.180.degree. ) is obtained between the photoelectric signal (light beat signal) detected from the interference light from the diffraction grating mark on the wafer by modulating its intensity with the beat frequency, and the light beat signal of the reference interference light produced separately from the two light carrier beams. Thus detecting any misregistrations within .+-.P/4 of the grating pitch P. Here, given a grating pitch P as 2 .mu.m (line-and-space of 1 .mu.m) and the resolution of the phase difference measurement as approximately 0.5.degree., the resolution for the misregisteration measurement will be (P/4).multidot.(0.5/180).perspectiveto.0.0014 .mu.m. Since a method of detecting mark positions such as this is of extremely high resolution, it is anticipated that the alignment precision is obtainable in the order of one digit high or more as-compared with the conventional mark position detection.
Now, in an alignment sensor of the kind, the intersecting angles of the two laser beams must be adjusted accurately so that a relational equation P=m.multidot.P' (m=1, 2, . . . ) is established between the grating pitch P and the pitch P' of the interference fringes and at the same time, the rotational error at the intersecting line of the plane including the major rays of light of the two laser beams with respect to the grating arrangement direction and the wafer surface must be set substantially at zero, that is, the interference fringes and diffraction grating must be set in parallel accurately. Otherwise it is impossible to utilize the advantage of high resolution sufficiently and there is a problem that the alignment precision can be lowered.
In the conventional art, therefore, the interference light from the grating mark is detected photoelectrically while changing the intersecting angles of the two laser beams. Then, there is obtained an intersecting angle at which the intensity of the interference light becomes the greatest, and the intersecting angles of the two laser beams are adjusted in order to set the pitch P' of the interference fringes accurately for the grating pitch P so that the above-mentioned relational equation can be satisfied.
On the other hand, as regards the rotational error at the intersecting line of the plane including the major rays of light of the two laser beams and the wafer surface, the interference light from the grating mark is sequentially detected photoelectrically while the diffraction grating mark (wafer) and interference fringes are being rotated relatively. Then, by rotating the grating mark and the interference fringes relatively to make the intensity of the interference light the greatest, the above-mentioned rotational error is set substantially at zero.
However, in the conventional art mentioned above, the maximum value of the diffraction light intensity (the voltage of light beat signal) is detected, that is, using a hill-climbing method, the intersecting angles or the rotational error of the two laser beams is measured and adjusted. In general, there is a fundamental problem for the hill-climbing method that the inclination (sensitivity) of signal changes is zero at the maximum value. Further, the intersecting angle and rotational error are calculated by monitoring its electrical level. It is therefore easier to receive the influence of noise or the like, and there is also a problem that a sufficiently accurate measurement cannot be obtained.